1: Quantitative Characteristics Vary Continuously and Many Are Influenced by Alleles at Multiple Loci
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Quantitative Genetics
(a) Discontinuous characteristic
1 A discontinuous (qualitative)
characteristic exhibits only
a few, easily distinguished
phenotypes.
2 The plants are either
dwarf or tall.
Number of individuals
Dwarf
Tall
all have one gene that encodes a plant hormone. These genotypes produce one dose of the hormone and a plant that is
11 cm tall. Even in this simple example of only three loci, the
relation between genotype and phenotype is quite complex.
The more loci encoding a characteristic, the greater the
complexity.
The influence of environment on a characteristic also
can complicate the relation between genotype and
Table 16.1
Phenotype (height)
Plant Genotype
Hypothetical example of plant
height determined by pairs of
alleles at each of three loci
Doses of Hormone
Height (cm)
0
10
1
11
2
12
3
13
4
14
5
15
6
16
(b) Continuous characteristic
4 The plants exhibit a
wide range of heights.
AϪAϪ BϪBϪ CϪCϪ
ϩ Ϫ
Ϫ Ϫ
Ϫ Ϫ
Ϫ Ϫ
ϩ Ϫ
Ϫ Ϫ
A A B B C C
A A B B C C
Number of individuals
3 A continuous (quantitative)
characteristic exhibits a
continuous range of
phenotypes.
AϪAϪ BϪBϪ CϪCϩ
AϩAϩ BϪBϪ CϪCϪ
Ϫ Ϫ
ϩ ϩ
Ϫ Ϫ
A A B B C C
AϪAϪ BϪBϪ CϩCϩ
AϩAϪ BϩBϪ CϪCϪ
AϩAϪ BϪBϪ CϩCϪ
Dwarf
Tall
Phenotype (height)
AϪAϪ BϩBϪ CϩCϪ
AϩAϩ BϩBϪ CϪCϪ
ϩ ϩ
Ϫ Ϫ
ϩ Ϫ
16.1 Discontinuous and continuous characteristics differ in
A A B B C C
the number of phenotypes exhibited.
AϩAϪ BϩBϩ CϪCϪ
For quantitative characteristics, the relation between
genotype and phenotype is often more complex. If the characteristic is polygenic, many different genotypes are possible,
several of which may produce the same phenotype. For
instance, consider a plant whose height is determined by
three loci (A, B, and C), each of which has two alleles.
Assume that one allele at each locus (Aϩ, Bϩ, and Cϩ)
encodes a plant hormone that causes the plant to grow 1 cm
above its baseline height of 10 cm. The other allele at each
locus (AϪ, BϪ, and CϪ) does not encode a plant hormone
and thus does not contribute to additional height. If we consider only the two alleles at a single locus, 3 genotypes are
possible (AϩAϩ, AϩAϪ, and AϪAϪ). If all three loci are taken
into account, there are a total of 33 ϭ 27 possible multilocus
genotypes (AϩAϩ BϩBϩ CϩCϩ, AϩAϪ BϩBϩ CϩCϩ, etc.).
Although there are 27 genotypes, they produce only seven
phenotypes (10 cm, 11 cm, 12 cm, 13 cm, 14 cm, 15 cm, and
16 cm in height). Some of the genotypes produce the same
phenotype (Table 16.1); for example, genotypes AϩAϪ
BϪBϪ CϪCϪ, AϪAϪ BϩBϪ CϪCϪ, and AϪAϪ BϪBϪ CϩCϪ
AϪAϪ BϩBϩ CϩCϪ
AϩAϪ BϪBϪ CϩCϩ
AϪAϪ BϩBϪ CϩCϩ
AϩAϪ BϩBϪ CϩCϪ
AϩAϩ BϩBϩ CϪCϪ
ϩ ϩ
ϩ Ϫ
ϩ Ϫ
A A B B C C
AϩAϪ BϩBϩ CϩCϪ
AϪAϪ BϩBϩ CϩCϩ
AϩAϩ BϪBϪ CϩCϩ
AϩAϪ BϩBϪ CϩCϩ
AϩAϩ BϩBϩ CϩCϪ
ϩ Ϫ
ϩ ϩ
ϩ ϩ
A A B B C C
AϩAϩ BϩBϪ CϩCϩ
AϩAϩ BϩBϩ CϩCϩ
Note: Each ϩ allele contributes 1 cm in height above a baseline of 10 cm.
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Chapter 16
Number of individuals
AA
Aa
aa
Dwarf
Tall
It is impossible to know whether an individual
with this phenotype is genotype AA or Aa.
16.2 For a quantitative characteristic, each genotype may
produce a range of possible phenotypes. In this hypothetical
example, the phenotypes produced by genotypes AA, Aa, and aa
overlap.
phenotype. Because of environmental effects, the same genotype may produce a range of potential phenotypes (the norm
of reaction; see p. 96 in Chapter 4). The phenotypic ranges
of different genotypes may overlap, making it difficult to
know whether individuals differ in phenotype because of
genetic or environmental differences (Figure 16.2).
In summary, the simple relation between genotype and
phenotype that exists for many qualitative (discontinuous)
characteristics is absent in quantitative characteristics, and it
is impossible to assign a genotype to an individual on the
basis of its phenotype alone. The methods used for analyzing qualitative characteristics (examining the phenotypic
ratios of progeny from a genetic cross) will not work with
quantitative characteristics. Our goal remains the same: we
wish to make predictions about the phenotypes of offspring
produced in a genetic cross. We may also want to know how
much of the variation in a characteristic results from genetic
differences and how much results from environmental differences. To answer these questions, we must turn to statistical methods that allow us to make predictions about the
inheritance of phenotypes in the absence of information
about the underlying genotypes.
may say that two people are both 5 feet 11 inches tall, but
careful measurement may show that one is slightly taller than
the other.
Some characteristics are not continuous but are nevertheless considered quantitative because they are determined
by multiple genetic and environmental factors. Meristic
characteristics, for instance, are measured in whole numbers. An example is litter size: a female mouse may have 4, 5,
or 6 pups but not 4.13 pups. A meristic characteristic has a
limited number of distinct phenotypes, but the underlying
determination of the characteristic may still be quantitative.
These characteristics must therefore be analyzed with the
same techniques that we use to study continuous quantitative characteristics.
Another type of quantitative characteristic is a threshold characteristic, which is simply present or absent. For
example, the presence of some diseases can be considered a
threshold characteristic. Although threshold characteristics
exhibit only two phenotypes, they are considered quantitative because they, too, are determined by multiple genetic
and environmental factors. The expression of the characteristic depends on an underlying susceptibility (usually
referred to as liability or risk) that varies continuously.
When the susceptibility is larger than a threshold value, a
specific trait is expressed (Figure 16.3). Diseases are often
threshold characteristics because many factors, both genetic
and environmental, contribute to disease susceptibility. If
enough of the susceptibility factors are present, the disease
develops; otherwise, it is absent. Although we focus on the
genetics of continuous characteristics in this chapter, the
same principles apply to many meristic and threshold
characteristics.
It is important to point out that just because a characteristic can be measured on a continuous scale does not
mean that it exhibits quantitative variation. One of the characteristics studied by Mendel was height of the pea plant,
which can be described by measuring the length of the
plant’s stem. However, Mendel’s particular plants exhibited
only two distinct phenotypes (some were tall and others
Types of Quantitative Characteristics
Before we look more closely at polygenic characteristics and
relevant statistical methods, we need to more clearly define
what is meant by a quantitative characteristic. Thus far, we
have considered only quantitative characteristics that vary
continuously in a population. A continuous characteristic can
theoretically assume any value between two extremes; the
number of phenotypes is limited only by our ability to precisely measure the phenotype. Human height is a continuous
characteristic because, within certain limits, people can theoretically have any height. Although the number of phenotypes possible with a continuous characteristic is infinite, we
often group similar phenotypes together for convenience; we
Threshold
Number of
individuals
410
Healthy
Diseased
Susceptibility to disease
16.3 Threshold characteristics display only two possible
phenotypes—the trait is either present or absent—but they
are quantitative because the underlying susceptibility to the
characteristic varies continuously. When the susceptibility
exceeds a threshold value, the characteristic is expressed.
Quantitative Genetics
short), and these differences were determined by alleles at a
single locus. The differences that Mendel studied were therefore discontinuous in nature.
Concepts
Characteristics whose phenotypes vary continuously are called
quantitative characteristics. For most quantitative characteristics,
the relation between genotype and phenotype is complex. Some
characteristics whose phenotypes do not vary continuously also
are considered quantitative because they are influenced by multiple genes and environmental factors.
P
Plants with
white kernels
ϫ
Plants with
purple kernels
T
F1
Plants with red kernels
T
1
16 plants with purple kernels
4
16 plants with dark-red kernels
6
16 plants with red kernels
F2
4
16 plants with light-red kernels
1
16 plants with white kernels
Polygenic Inheritance
After the rediscovery of Mendel’s work in 1900, questions
soon arose about the inheritance of continuously varying
characteristics. These characteristics had already been the
focus of a group of biologists and statisticians, led by Francis
Galton, who used statistical procedures to examine the
inheritance of quantitative characteristics such as human
height and intelligence. The results of these studies showed
that quantitative characteristics are inherited, although the
mechanism of inheritance was not yet known. Some biometricians argued that the inheritance of quantitative characteristics could not be explained by Mendelian principles,
whereas others felt that Mendel’s principles acting on
numerous genes (polygenes) could adequately account for
the inheritance of quantitative characteristics.
This conflict began to be resolved through independent
work by Wilhelm Johannsen, George Udny Yule, and
Herman Nilsson-Ehle, who each studied continuous variation in plants. The argument was finally laid to rest in 1918,
when Ronald Fisher demonstrated that the inheritance of
quantitative characteristics could indeed be explained by the
cumulative effects of many genes, each following Mendel’s
rules.
Kernel Color in Wheat
To illustrate how multiple genes acting on a characteristic
can produce a continuous range of phenotypes, let us examine one of the first demonstrations of polygenic inheritance.
Nilsson-Ehle studied kernel color in wheat and found that
the intensity of red pigmentation was determined by three
unlinked loci, each of which had two alleles.
Nilsson-Ehle obtained several homozygous varieties of
wheat that differed in color. Like Mendel, he performed
crosses between these homozygous varieties and studied the
ratios of phenotypes in the progeny. In one experiment, he
crossed a variety of wheat that possessed white kernels with
a variety that possessed purple (very dark red) kernels and
obtained the following results:
Nilsson-Ehle interpreted this phenotypic ratio as the
result of the segregation of alleles at two loci. (Although he
found alleles at three loci that affect kernel color, the two
varieties used in this cross differed at only two of the loci.)
He proposed that there were two alleles at each locus: one
that produced red pigment and another that produced no
pigment. We’ll designate the alleles that encoded pigment
Aϩ and Bϩ and the alleles that encoded no pigment AϪ and
BϪ. Nilsson-Ehle recognized that the effects of the genes
were additive. Each gene seemed to contribute equally to
color; so the overall phenotype could be determined by
adding the effects of all the genes, as shown in the following table.
Genotype
AϩAϩ BϩBϩ
Doses of pigment
4
Phenotype
purple
A+ A+ B + B f
A+ A- B + B +
3
dark red
A+ A+ B - B A- A- B + B +
A+ A- B + B -
f
2
red
A+ A- B - B f
A- A- B + B -
1
light red
AϪAϪ BϪBϪ
0
white
Notice that the purple and white phenotypes are each
encoded by a single genotype, but other phenotypes may
result from several different genotypes.
From these results, we see that five phenotypes are possible when alleles at two loci influence the phenotype and the
effects of the genes are additive. When alleles at more than
two loci influence the phenotype, more phenotypes are possible, and the color would appear to vary continuously
between white and purple. If environmental factors had
influenced the characteristic, individuals of the same genotype would vary somewhat in color, making it even more difficult to distinguish between discrete phenotypic classes.
Luckily, environment played little role in determining kernel
color in Nilsson-Ehle’s crosses, and only a few loci encoded
411
412
Chapter 16
color; so Nilsson-Ehle was able to distinguish among the different phenotypic classes. This ability allowed him to see the
Mendelian nature of the characteristic.
Let’s now see how Mendel’s principles explain the ratio
obtained by Nilsson-Ehle in his F2 progeny. Remember that
Nilsson-Ehle crossed the homozygous purple variety (AϩAϩ
BϩBϩ) with the homozygous white variety (AϪAϪ BϪBϪ),
producing F1 progeny that were heterozygous at both loci
(AϩAϪ BϩBϪ). This is a dihybrid cross, like those that we
worked in Chapter 3, except that both loci encode the same
trait. All the F1 plants possessed two pigment-producing
alleles that allowed two doses of color to make red kernels.
The types and proportions of progeny expected in the F2 can
be found by applying Mendel’s principles of segregation and
independent assortment.
Let’s first examine the effects of each locus separately. At
the first locus, two heterozygous F1s are crossed (AϩAϪ ϫ
AϩAϪ). As we learned in Chapter 3, when two heterozygotes
are crossed, we expect progeny in the proportions 14 AϩAϩ,
1
2 AϩAϪ, and 14 AϪAϪ. At the second locus, two heterozygotes also are crossed, and, again, we expect progeny in the
proportions 14 BϩBϩ, 12 BϩBϪ, and 14 BϪBϪ.
To obtain the probability of combinations of genes at
both loci, we must use the multiplication rule of probability
(see Chapter 3), which is based on Mendel’s principle of
independent assortment. The expected proportion of F2
progeny with genotype AϩAϩ BϩBϩ is the product of the
probability of obtaining genotype AϩAϩ (14) and the probability of obtaining genotype BϩBϩ (14), or 14 ϫ 14 ϭ 116
(Figure 16.4). The probabilities of each of the phenotypes
can then be obtained by adding the probabilities of all the
genotypes that produce that phenotype. For example, the red
phenotype is produced by three genotypes:
Genotype
AϩAϩ BϪBϪ
AϪAϪ BϩBϩ
AϩAϪ BϩBϪ
Experiment
Question: How is a continous trait, such as kernel color
in wheat, inherited?
Methods
Cross wheat having white kernels and wheat
having purple kernels. Intercross the F1 to
produce F2.
P generation
A+ A+ B+ B+
Purple
Results
White
16.4 Nilsson-Ehle demonstrated that kernel color in wheat
is inherited according to Mendelian principles. He crossed two
varieties of wheat that differed in pairs of alleles at two loci affecting
kernel color. A purple strain (AϩAϩ BϩBϩ) was crossed with a white
strain (AϪAϪ BϪBϪ), and the F1 was intercrossed to produce F2
progeny. The ratio of phenotypes in the F2 can be determined by
breaking the dihybrid cross into two simple single-locus crosses
and combining the results by using the multiplication rule.
Red
Break into simple crosses
A+ A– ןA+ A–
B+ B– ןB+ B–
1/4 A + A + 1/2 A + A – 1/4 A –
A – 1/4 B + B + 1/2 B + B – 1/4 B – B –
Combine results
F2 generation
1/4 B + B +
1/4 A + A +
1/2 B + B –
1/4 B –
B–
1/4 B + B +
1/2 A + A –
A–
1/4ן1/4 = 1/16
4
Purple
1/4ן1/2 = 2/16
3
Dark
red
1/4ן1/4 = 1/16
2
Red
1/2ן1/4 = 2/16
3
Dark
red
A+ A+ B+ B+
A+ A+ B+ B–
A+ A+ B– B–
A+ A– B+ B+
1/2ן1/2 = 4/16
A+ A– B+ B–
2
Red
1/4 B –
1/2ן1/4 = 2/16
+ – – –
1
Light
red
1/4ן1/4 = 1/16
2
Red
B–
1/4 B + B +
1/4 A –
Number of
Phenopigment genes type
1/2 B + B –
1
Thus, the overall probability of obtaining red kernels in the
F2 progeny is 116 ϩ 116 ϩ 14 ϭ 616. Figure 16.4 shows that
the phenotypic ratio expected in the F2 is 116 purple, 416
dark red, 616 red, 416 light red, and 116 white. This phenotypic ratio is precisely what Nilsson-Ehle observed in his
F2 progeny, demonstrating that the inheritance of a
A– A– B– B–
F1 generation
A+ A– B+ B–
Probability
16
1
16
1
4
ן
A A B B
A– A– B+ B+
1/2 B + B –
1/4ן1/2 = 2/16
A– A– B+ B–
1
Light
red
1/4 B –
1/4ן1/4 = 1/16
A– A– B– B–
0
White
B–
Combine common phenotypes
F2 ratio
Number of
Frequency pigment genes
Phenotype
1/16
4
Purple
4/16
3
Dark red
6/16
2
Red
4/16
1
Light red
1/16
0
White
Conclusion: Kernel color in wheat is inherited according to
Mendel’s principles acting on alleles at two loci.
Quantitative Genetics
One locus, Aa ןAa
Relative number of progeny
Two loci, Aa Bb ןAa Bb
1 As the number of
loci affecting the
trait increases,…
phenotypic classes. Second, the genes affecting kernel color
had strictly additive effects, making the relation between
genotype and phenotype simple. Third, environment played
almost no role in the phenotype; had environmental factors
modified the phenotypes, distinguishing between the five
phenotypic classes would have been difficult. Finally, the loci
that Nilsson-Ehle studied were not linked; so the genes
assorted independently. Nilsson-Ehle was fortunate: for
many polygenic characteristics, these simplifying conditions
are not present and Mendelian inheritance of these characteristics is not obvious.
Concepts
The principles that determine the inheritance of quantitative characteristics are the same as the principles that determine the inheritance of discontinuous characteristics, but more genes take part
in the determination of quantitative characteristics.
Five loci,
Aa Bb Cc Dd Ee ןAa Bb Cc Dd Ee
16.2 Analyzing Quantitative
2 …the number
of phenotypic
classes increases.
Phenotype classes
16.5 The results of crossing individuals heterozygous for
different numbers of loci affecting a characteristic.
continuously varying characteristic such as kernel color is
indeed according to Mendel’s basic principles.
Nilsson-Ehle’s crosses demonstrated that the difference
between the inheritance of genes influencing quantitative
characteristics and the inheritance of genes influencing discontinuous characteristics is in the number of loci that determine the characteristic. When multiple loci affect a
character, more genotypes are possible; so the relation
between the genotype and the phenotype is less obvious. For
example, in a cross of F1 individuals heterozygous for alleles
at a single locus with additive effects, 3 phenotypes appear
among the progeny (Figure 16.5). When parents of the cross
are heterozygous at two loci, there are 5 phenotypes in the
progeny, and, when the parents are heterozygous at five loci,
there are 11 phenotypes in the progeny. As the number of
loci affecting a character increases, the number of phenotypic classes in the F2 increases.
Several conditions of Nilsson-Ehle’s crosses greatly simplified the polygenic inheritance of kernel color and made it
possible for him to recognize the Mendelian nature of the
characteristic. First, genes affecting color segregated at only
two or three loci. If genes at many loci had been segregating,
he would have had difficulty in distinguishing the
Characteristics
Because quantitative characteristics are described by a measurement and are influenced by multiple factors, their inheritance must be analyzed statistically.
Distributions
Understanding the genetic basis of any characteristic begins
with a description of the numbers and kinds of phenotypes
present in a group of individuals. Phenotypic variation in a
group, such as the progeny of a cross, can be conveniently
represented by a frequency distribution, which is a graph of
the frequencies (numbers or proportions) of the different
phenotypes (Figure 16.6). In a typical frequency distribution, the phenotypic classes are plotted on the horizontal (x)
Number of individuals
Many loci
Phenotype (body weight)
16.6 A frequency distribution is a graph that displays the
number or proportion of different phenotypes. Phenotypic values
are plotted on the horizontal axis, and the numbers (or proportions)
of individuals in each class are plotted on the vertical axis.
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Chapter 16
20
(b) Squash fruit length
(c) Earwig forceps length
2 The distribution of
fruit length among
the F2 progeny is
skewed to the right.
10
3 A distribution
with two
peaks is bimodal.
Frequency (%)
1 This type of
symmetrical
(bell-shaped)
distribution is
called a normal
distribution.
20
10
12 13 14 15 16 17 18 19%
30
20
10
4
6
8 10 12 14 16 18 20 cm
3
4
5
6
7
8
9 mm
16.7 Distributions of phenotypes can assume several different shapes.
axis, and the numbers (or proportions) of individuals in
each class are plotted on the vertical (y) axis. A frequency distribution is a concise method of summarizing all phenotypes
of a quantitative characteristic.
Connecting the points of a frequency distribution with
a line creates a curve that is characteristic of the distribution.
Many quantitative characteristics exhibit a symmetrical
(bell-shaped) curve called a normal distribution (Figure
16.7a). Normal distributions arise when a large number of
independent factors contribute to a measurement, as is often
the case in quantitative characteristics. Two other common
types of distributions (skewed and bimodal) are illustrated
in Figure 16.7b and c.
Suppose we have five measurements of height in centimeters: 160, 161, 167, 164, and 165. If we represent a group of measurements as x1, x2, x3, and so forth, then the mean (x) is
calculated by adding all the individual measurements and dividing by the total number of measurements in the sample (n):
x =
x1 + x2 + x3 + Á + xn
n
(16.1)
In our example, x1 ϭ 160, x2 ϭ 161, x3 ϭ 167, and so forth.
The mean height (x) equals:
x =
160 + 161 + 167 + 164 + 165
817
=
= 163.4
5
5
A shorthand way to represent this formula is
The Mean
The mean, also called the average, provides information
about the center of the distribution. If we measured the
heights of 10-year-old and 18-year-old boys and plotted a
frequency distribution for each group, we would find that
both distributions are normal, but the two distributions
would be centered at different heights, and this difference
would be indicated in their different means (Figure 16.8).
x =
a xi
n
(16.2)
x =
1
x
na i
(16.3)
or
where the symbol © means “the summation of ” and xi represents individual x values.
x = 135 cm
x = 175 cm
10-year-old boys
18-year-old boys
50
Percentage
Frequency (%)
(a) Sugar beet percentage of sucrose
Frequency (%)
414
25
0
110
120
130
140
150
160
Height (cm)
170
16.8 The mean provides information about the center of a distribution. Both distributions
of heights of 10-year-old and 18-year-old boys are normal, but they have different locations along a
continuum of height, which makes their means different.
180
190
200
Quantitative Genetics
Mean (x)
Concepts
s 2 = 0.25
Frequency
The greater the
variance, the more
spread out the
distribution is
about the mean.
s 2 = 1.0
s 2 = 4.0
5
6
7
8
9
10 11
Length
12
13
14
15
16.9 The variance provides information about the
variability of a group of phenotypes. Shown here are three
distributions with the same mean but different variances.
The Variance
A statistic that provides key information about a distribution is the variance, which indicates the variability of a
group of measurements, or how spread out the distribution
is. Distributions may have the same mean but different
variances (Figure 16.9). The larger the variance, the greater
the spread of measurements in a distribution about its
mean.
The variance (s2) is defined as the average squared deviation from the mean:
a (xi - x)
n - 1
2
s2 =
(16.4)
To calculate the variance, we (1) subtract the mean from each
measurement and square the value obtained, (2) add all the
squared deviations, and (3) divide this sum by the number
of original measurements minus 1. For example, suppose we
wanted to calculate the variance for the five heights mentioned earlier (160, 161, 167, 164, and 165 cm). As already
shown, the mean of these heights is 163.4 cm. The variance
for the heights is:
s2
(160 - 163.4)2 + (161 - 163.4)2 + (167 - 163.4)2
+ (164 - 163.4)2 + (165 - 163.4)2
=
5-1
(-3.4)2 + (-2.4)2 + (3.6)2 + (0.6)2 + (1.6)2
4
11.56 + 5.76 + 12.96 + 0.36 + 2.56
=
4
= 8.3
=
The mean and variance describe a distribution of measurements:
the mean provides information about the location of the center of
a distribution, and the variance provides information about its
variability.
Applying Statistics to the Study
of a Polygenic Characteristic
Edward East carried out one early statistical study of polygenic inheritance on the length of flowers in tobacco
(Nicotiana longiflora). He obtained two varieties of tobacco
that differed in flower length: one variety had a mean flower
length of 40.5 mm, and the other had a mean flower length
of 93.3 mm (Figure 16.10). These two varieties had been
inbred for many generations and were homozygous at all loci
contributing to flower length. Thus, there was no genetic
variation in the original parental strains; the small differences in flower length within each strain were due to environmental effects on flower length.
When East crossed the two strains, he found that flower
length in the F1 was about halfway between that in the two
parents (see Figure 16.10), as would be expected if the genes
determining the differences in the two strains were additive
in their effects. The variance of flower length in the F1 was
similar to that seen in the parents because all the F1 had the
same genotype, as did each parental strain (the F1 were all
heterozygous at the genes that differed between the two
parental varieties).
East then interbred the F1 to produce F2 progeny. The
mean flower length of the F2 was similar to that of the F1, but
the variance of the F2 was much greater (see Figure 16.10).
This greater variability indicates that not all of the F2 progeny had the same genotype.
East selected some F2 plants and interbred them to produce F3 progeny. He found that flower length of the F3
depended on flower length in the plants selected as their parents. This finding demonstrated that flower-length differences in the F2 were partly genetic and were therefore passed
to the next generation.
16.3 Heritability Is Used to
Estimate the Proportion
of Variation in a Trait
That Is Genetic
In addition to being polygenic, quantitative characteristics
are frequently influenced by environmental factors. It is
often useful to know how much of the variation in a quantitative characteristic is due to genetic differences and how
much is due to environmental differences. The proportion of
the total phenotypic variation that is due to genetic differences is known as the heritability.
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Chapter 16
Experiment
Question: How is flower length in tobacco plants inherited?
Flower length
Methods
P generation
Parental
strain B
Frequency
Frequency
Parental
strain A
31 34 37 40 43 46
Flower length
x = 40.5 mm
F1 generation
84 87 90 93 96 99 102
Flower length
x = 93.3 mm
1 Flower length in the F1 was
about halfway between that
in the two parents,…
Frequency
Results
55 58 61 64 67 70 73
Flower length
2 …and the variance in the F1
was similar to that seen in the
parents.
F2 generation
Frequency
416
3 The mean of the F2 was similar
to that observed for the F1,…
70
60
50
40
30
20
10
0
52 55 58 61 64 67 70 73 76 79 82 85 88
Flower length (mm)
4 …but the variance in the F2
was greater, indicating the
presence of different genotypes among the F2 progeny.
Conclusion: Flower length of the F1 and F2 is consistent
with the hypothesis that the characteristic is determined by
several genes that are additive in their effects.
16.10 Edward East conducted an early statistical study of
the inheritance of flower length in tobacco.
Consider a dairy farmer who owns several hundred milk
cows. The farmer notices that some cows consistently produce more milk than others. The nature of these differences
is important to the profitability of his dairy operation. If the
differences in milk production are largely genetic in origin,
then the farmer may be able to boost milk production by
selectively breeding the cows that produce the most milk. On
the other hand, if the differences are largely environmental
in origin, selective breeding will have little effect on milk
production, and the farmer might better boost milk production by adjusting the environmental factors associated with
higher milk production. To determine the extent of genetic
and environmental influences on variation in a characteristic, phenotypic variation in the characteristic must be partitioned into components attributable to different factors.
Phenotypic Variance
To determine how much of phenotypic differences in a population is due to genetic and environmental factors, we must
first have some quantitative measure of the phenotype under
consideration. Consider a population of wild plants that differ in size. We could collect a representative sample of plants
from the population, weigh each plant in the sample, and
calculate the mean and variance of plant weight. This phenotypic variance is represented by VP.
Components of phenotypic variance First, some of the
phenotypic variance may be due to differences in genotypes
among individual members of the population. These differences are termed the genetic variance and are represented
by VG.
Second, some of the differences in phenotype may be
due to environmental differences among the plants; these
differences are termed the environmental variance, VE.
Environmental variance includes differences in environmental factors such as the amount of light or water that the plant
receives; it also includes random differences in development
that cannot be attributed to any specific factor. Any variation
in phenotype that is not inherited is, by definition, a part of
the environmental variance.
Third, genetic–environmental interaction variance
(VGE) arises when the effect of a gene depends on the specific environment in which it is found. An example is shown
in Figure 16.11. In a dry environment, genotype AA produces a plant that averages 12 g in weight, and genotype aa
produces a smaller plant that averages 10 g. In a wet environment, genotype aa produces the larger plant, averaging 24 g
in weight, whereas genotype AA produces a plant that averages 20 g. In this example, there are clearly differences in the
two environments: both genotypes produce heavier plants in
the wet environment. There are also differences in the
weights of the two genotypes, but the relative performances
of the genotypes depend on whether the plants are grown in
a wet or a dry environment. In this case, the influences on